In critical control circuits it has become standard practice to require redundant response from sensing devices and associated components before a critical control action can be taken. This provides relative certainty that the control action is only in response to valid signals and is not a result of failure of the sensor itself or of some other associated component or device. For example, in the field of monitoring and controlling the rotational speed of a turbine rotor, it is necessary to take very positive action in the event the rotational speed reaches a critical overspeed limit. It is common practice, therefore, to provide at least three separate and independent channels by which rotational speed is monitored. For these three-channel monitoring and control systems, redundant logic switching networks have been provided which require that at least two out of the three channels must respond properly before control action can follow. Such logic control systems are described in U.S. Pat. No. 3,748,540 to Eggenberger et al, the disclosure of which is incorporated herein by reference thereto.
A problem with the aforementioned redundant logic control networks has been that a malfunction within any one of the separate sensor/control channels can destroy the supposed redundancy without being noticed. Under these conditions, a failure, false response, or even a test of any one of the remaining channels can lead to strong control action being taken on a nonvalid signal.
The above cited patent to Eggenberger et al seeks to alleviate this problem by providing a redundancy monitoring technique wherein the switch contacts within the redundant control network are provided with parallel connected voltage sensing devices which respond upon failure of certain of the redundant contacts and can therefore be used to notify operating personnel of an abnormal condition. Close inspection of the system reveals, however, that the technique is directed to use with actively open switch contacts, i.e, those switches which become closed upon sensing a limit condition. Furthermore, the voltage sensors actually used in practice have been meter relays; a device well known to those of ordinary skill in the art and known to require a relatively long time before they respond.
Accordingly, it is one object of the present invention to provide a monitor for redundant logic control networks which can be used to signal the loss of such redundancy in an actively closed (closed under normal operating conditions) redundant logic network, and which indicates with particularity that portion of the control network which is inoperable.
It is a further object of the invention to provide a monitor for redundant logic control networks wherein the monitor has improved response time and which is responsive to changes in current flow within the control network.